Herschel finds a dust belt surrounding a subgiant planetary system

Sen—ESA's Herschel space observatory has obtained the first images of a dust belt, produced by colliding comets or asteroids, orbiting a subgiant star known to host a planetary system.

Stars like our Sun exhaust their central hydrogen fuel reserve and start burning it in shells around the core, swelling to become subgiant stars, before becoming red giants. Kappa Coronae Borealis (Kappa Cor Bor) is 1.5 solar masses and around 2.5 billion years old. The star lies about 100 light years away. It is known to host one giant planet roughly twice the mass of Jupiter orbiting at a distance equivalent to the Asteroid Belt in our own Solar System. A second planet is suspected, but its mass is not well constrained.

During the subgiant phase, planets, asteroids and comet belts around these 'retired' stars are expected to survive, but observations are needed to measure their properties. One approach is to search for discs of dust around the stars, generated by collisions between populations of asteroids or comets.

Using the sensitive far-infrared detection capabilities of Herschel, astronomers have been able to resolve bright emission around Kappa Cor Bor, indicating the presence of a dusty debris disc.

"This is the first 'retired' star that we have found with a debris disc and one or more planets," says Amy Bonsor of the Institute de Planetologie et d'Astrophysique de Grenoble, and lead author of the study.

"The disc has survived the star's entire lifetime without being destroyed. That's very different to our own Solar System, where most of the debris was cleared away in a phase called the Late Heavy Bombardment era, around 600 million years after the Sun formed."

Dr Bonsor's team propose three possible configurations for the disc and planets that fit Herschel’s observations of Kappa Cor Bor.

The first model has just one continuous dust belt extending from 20 Astronomical Units (AU) to 220 AU. By comparison, the Kuiper Belt in our Solar System spans a narrower range of distances, 30 to 50 AU from the Sun. One of the planets orbits at a distance of greater than 7 AU from the star, and its gravitational influence may sculpt the inner edge of the disc.

The second model has the disc being stirred by the gravitational influence of both companions, so that the rate of dust production in the disc peaks at around 70 to 80 AU from the star.

In the third model, the dust disc is in two narrow belts, centred on 40 AU and 165 AU, respectively. Here, the outermost companion, possibly a substellar brown dwarf, may orbit between the two belts at a distance of about 7 AU and 70 AU.

This is the first known example of a subgiant star with planets and a debris disc orbiting it, so more examples are needed to determine whether it is unusual or not.

"Thanks to Herschel's sensitive far-infrared capabilities and its rich dataset, we already have hints of other subgiant stars that may also have dusty discs. More work will be needed to see if they also have planets," says Goran Pilbratt, ESA's Herschel project scientist.